Field of the Invention
The present invention relates to an organic light emitting display device, and more particularly, to an organic light emitting display device for improving a contrast ratio.
Discussion of the Related Art
Recently, with the advancement of multimedia, the importance of flat panel display (FPD) devices is increasing. Therefore, various FPD devices such as liquid crystal display (LCD) devices, plasma display panel (PDP) devices, and organic light emitting display devices are being used practically. In such FPD devices, the organic light emitting display devices have a fast response time of 1 ms or less and low power consumption, and have no limitation in a viewing angle because the organic light emitting display devices self-emit light. Accordingly, the organic light emitting display devices are attracting much attention as next generation FPD devices.
A related art organic light emitting display device includes a plurality of pixels that display an image. Each of the plurality of pixels includes an organic light emitting layer, which is formed between an anode electrode and a cathode electrode, and a pixel circuit that emits light from an organic light emitting device. The pixel circuit includes a switching transistor, a driving transistor, and a capacitor. The switching transistor is turned on according to a switching signal to supply a data voltage to the driving transistor. The driving transistor is turned on with the data voltage supplied from the switching transistor, and controls a current flowing to the organic light emitting device to control emission of light from the organic light emitting device. The capacitor stores a voltage between a gate electrode and source electrode of the driving transistor, and turns on the driving transistor by using the stored voltage. The organic light emitting device emits light with a current supplied from the driving transistor.
However, in the related art organic light emitting display device, a characteristic difference (such as a threshold voltage (Vth) and mobility) of the driving transistor occurs due to a process differential, and for this reason, the amount of current driving the organic light emitting device is changed, causing a luminance deviation between pixels. Generally, the characteristic difference of the driving transistor causes a smear or a pattern to a screen, and a characteristic difference due to a deterioration of the organic light emitting device which is caused by long-time driving reduces a service life of an organic light emitting display panel or causes image sticking. As a method for solving a problem caused by a characteristic change of each pixel, internal compensation technology and external compensation technology are known.
The internal compensation technology adds a compensation circuit (including at least one compensation transistor and at least one compensation capacitor) into a pixel circuit of each pixel, and compensates for a characteristic change of each pixel by using a compensation circuit. However, in the internal compensation technology, an aperture ratio of each pixel is reduced due to a compensation transistor, a compensation capacitor, and a signal line which are added into each pixel.
The external compensation technology senses a characteristic change of a pixel from the outside of the pixel, and reflects the sensed characteristic change in data of the pixel to compensate for the characteristic change of the pixel. The external compensation technology is disclosed in a related art reference such as Korean Patent Publication No. 10-2013-0066449.
The related art reference, as illustrated in FIG. 1, applies a reference voltage Vref to a gate electrode n1 of a driving transistor DT through a first transistor M1 and simultaneously applies a data voltage to a source electrode n2 of the driving transistor DT through a second transistor M2, thereby emitting light from an organic light emitting device OLED. In external compensation, the related art reference applies the reference voltage Vref to the gate electrode n1 of the driving transistor DT through the first transistor M1, and senses a characteristic change of the driving transistor and/or a characteristic change of the organic light emitting device OLED through a data line D which is connected to the source electrode n2 of the driving transistor DT through the second transistor M2.
The reference voltage Vref of the related art reference is fixed as a constant direct current (DC) voltage value. Therefore, the related art reference applies a low data voltage to the source electrode n2 of the driving transistor DT so as to realize a high gray scale, and applies a high data voltage to the source electrode n2 of the driving transistor DT so as to realize a low gray scale. Therefore, in the related art reference, the data voltage is applied to the source electrode n2 of the driving transistor DT connected to the organic light emitting device OLED, and thus, when the data voltage applied to the source electrode n2 of the driving transistor DT is higher than a threshold voltage (Vth_OLED) of the organic light emitting device OLED, the organic light emitting device OLED emits light with the data voltage during a data charging period of a pixel.
FIG. 2 is a waveform diagram showing a data voltage of a unit pixel for realizing a high gray scale and a low gray scale in one unit pixel, in the related art reference.
In the related art reference, a data voltage for realizing a high gray scale and a low gray scale will be described with common reference to FIGS. 1 and 2.
First, it is assumed that the reference voltage Vref supplied to each pixel is fixed as 16V and the threshold voltage Vth_oled of the organic light emitting device OLED is 6V, and it is assumed that a high gray scale having relatively high luminance is realized in a first horizontal period 1H and a low gray scale having relatively low luminance is realized in a second horizontal period 2H.
During a data charging period of the first horizontal period 1H, a red data voltage Vdata_R of 2V, a green data voltage Vdata_G of 4V, a blue data voltage Vdata_B of 8V, and a white data voltage Vdata_W of 6V are respectively applied to the source electrodes n2 of the driving transistors DT included in respective pixels R, G, B and W. Therefore, during a data charging period of the first horizontal period 1H, a voltage (Vref−Vdata_R) of 14V, a voltage (Vref−Vdata_G) of 12V, a voltage (Vref−Vdata_B) of 8V, and a voltage (Vref−Vdata_W) of 10V are each applied between the gate electrode and source electrode of the driving transistor DT included in a corresponding pixel among the pixels R, G, B and W. Thus, in an emission period of the first horizontal period 1H, the organic light emitting device OLED of each of the pixels R, G, B and W emits light with a data current corresponding to a corresponding voltage among the voltages 14V, 12V, 8V and 10V, thereby realizing a high gray scale having high luminance. Here, in the data charging period of the first horizontal period 1H, since the blue data voltage Vdata_B applied to the blue pixel is higher than the threshold voltage Vth_oled of the organic light emitting device OLED, the organic light emitting device OLED of the blue pixel emits light during the data charging period, thereby increasing a luminance of a high gray scale.
On the other hand, during a data charging period of the second horizontal period 2H, a red data voltage Vdata_R of 16V, a green data voltage Vdata_G of 12V, a blue data voltage Vdata_B of 14V, and a white data voltage Vdata_W of 14V are respectively applied to the source electrodes n2 of the driving transistors DT included in respective pixels R, G, B and W. Therefore, during a data charging period of the second horizontal period 2H, a voltage (Vref−Vdata_R) of 0V, a voltage (Vref−Vdata_G) of 4V, a voltage (Vref−Vdata_B) of 2V, and a voltage (Vref−Vdata_W) of 2V are each applied between the gate electrode and source electrode of the driving transistor DT included in a corresponding pixel among the pixels R, G, B and W. Thus, in an emission period of the second horizontal period 1H, the organic light emitting device OLED of each of the pixels R, G, B and W emits light with a data current corresponding to a corresponding voltage among the voltages 0V, 4V, 2V and 2V, thereby realizing a low gray scale having low luminance Here, in the data charging period of the first horizontal period 1H, since the data voltages Vdata_R, Vdata_G, Vdata_B and Vdata_W respectively applied to the pixels R, G, B and W are higher than the threshold voltage Vth_oled of the organic light emitting device OLED, all the organic light emitting devices OLED of the pixels R, G, B and W emit light during the data charging period, thereby increasing a luminance of a low gray scale.
For this reason, in the related art reference, a luminance of a low gray scale increases due to undesired emission of light from the organic light emitting device OLED in realizing a low gray scale, causing a reduction in a contrast ratio.